Distance Effect in Transient Thermography for Internal Defects Detection in Composites

Composite materials are commonly used in industrial systems for their superior physical and chemical properties. Due to the necessity for safety distance in industrial systems, non-destructive testing (NDT) for composites at long distances provides an important guarantee for prompt detection of internal equipment failures. The existing long-distance inspection methods are mostly passive NDT techniques. In this paper, a long-distance NDT method based on transient thermography is proposed. A theoretical model is established that integrates the transient thermography approach with the thermographic distance effect, providing the theoretical foundation for research based on distance detection effects. Different thermal wave features are extracted based on thermal physical parameters in the time and frequency domains, and the detection effect is compared based on different feature images. By conducting experiments on SIR–GFE bonded plate with artificially simulated internal defects, the effectiveness of both pulse thermography (PT) and step heating thermography (SHT) in long-distance inspection was compared, in which the SHT method was superior. The detection effect of feature images at different distances are compared using the SHT technique, the model's validity is tested, and the law of the function of distance in transient thermal imaging detection is derived. This work introduces the influence of distance in transient thermography for the first time, and it is hoped that this research can effectively guide and improve real industrial inspection.